I. Field
The present invention relates generally to circuits, and more specifically to a digitally programmable capacitor bank for use in a voltage controlled oscillator (VCO) and other circuits.
II. Background
VCOs are an integral part of many electronics circuits and are particularly important in communication circuits. For example, VCOs are often used to generate local oscillator (LO) signals used by transmitter and receiver subsystems for frequency upconversion and downconversion, respectively. VCOs are also used to generate clock signals for synchronous circuits (e.g., flip-flops). A wireless device (e.g., a cellular phone) in a wireless communication system may employ multiple VCOs to generate LO signals for transmitter and receiver circuitry and clock signals for digital circuitry.
A VCO is typically designed to operate at a specific frequency or a specific range of frequencies. The frequency of oscillation for the VCO may deviate from a nominal design value due to various factors such as circuit component tolerances, integrated circuit (IC) process variations, circuit component aging, and so on. The oscillation frequency may also change during normal operation due to various factors such as temperature, power supply variation, and so on. The VCO typically employs one or more variable capacitors (varactors) used to adjust the oscillation frequency in order to operate the VCO at the desired frequency. The tuning range of the VCO refers to the range of oscillation frequencies achieved by varying the varactor(s) across the entire adjustment range. The VCO may be designed with a sufficiently wide tuning range to cover the expected deviation in oscillation frequency due to all of the factors noted above. However, a wide tuning range may degrade phase noise performance. Phase noise refers to short-term random frequency fluctuations of an oscillator signal and is a parameter used to describe the quality of the oscillator signal.
To reduce the tuning range requirement, a VCO may employ a programmable capacitor bank to aid with the adjustment of the oscillation frequency. The capacitor bank contains a bank of tuning capacitors that may be individually switched on or off. Each tuning capacitor reduces the oscillation frequency when switched on. The tuning capacitors typically have binary weighted capacitances, which means that the first tuning capacitor in the bank has unit capacitance and each subsequent tuning capacitor has twice the capacitance of the preceding tuning capacitor. Different total tuning capacitances, in progressively larger discrete steps, may be obtained by switching on the appropriate tuning capacitor(s). A capacitance step is the difference in tuning capacitance for two adjacent discrete steps. With binary weighted tuning capacitors, the capacitance step may vary widely depending on which capacitors in the bank are switched on. A large variation in capacitance steps can complicate frequency adjustment for the VCO and degrade phase noise performance.
There is therefore a need in the art for a programmable capacitor bank that can provide improved performance for a VCO and other circuits.